Why Antarctic sea ice cover has increased under the effects of climate change

The first direct evidence that marked changes to Antarctic sea ice drift have occurred over the last 20 years, in response to changing winds, is published this week in the journal Nature Geoscience. Scientists from NERC's British Antarctic Survey (BAS) and NASA's Jet Propulsion Laboratory (JPL), Pasadena California explain why, unlike the dramatic losses reported in the Arctic, the Antarctic sea ice cover has increased under the effects of climate change.

Maps created by JPL using over 5 million individual daily ice motion measurements captured over a period of 19 years by four US Defense Meteorological satellites show, for the first time, the long-term changes in sea ice drift around Antarctica.

Lead author, Dr Paul Holland of BAS says: “Until now these changes in ice drift were only speculated upon, using computer models of Antarctic winds. This study of direct satellite observations shows the complexity of climate change. The total Antarctic sea-ice cover is increasing slowly, but individual regions are actually experiencing much larger gains and losses that are almost offsetting each other overall. We now know that these regional changes are caused by changes in the winds, which in turn affect the ice cover through changes in both ice drift and air temperature. The changes in ice drift also suggest large changes in the ocean surrounding Antarctica, which is very sensitive to the cold and salty water produced by sea-ice growth.”

“Sea ice is constantly on the move; around Antarctica the ice is blown away from the continent by strong northward winds. Since 1992 this ice drift has changed. In some areas the export of ice away from Antarctica has doubled, while in others it has decreased significantly.”

Sea ice plays a key role in the global environment - reflecting heat from the sun and providing a habitat for marine life. At both poles sea ice cover is at its minimum during late summer. However, during the winter freeze in Antarctica this ice cover expands to an area roughly twice the size of Europe. Ranging in thickness from less than a metre to several metres, the ice insulates the warm ocean from the frigid atmosphere above.

The new research also helps explain why observed changes in the amount of sea-ice cover are so different in the two Polar Regions. The Arctic has experienced dramatic ice losses in recent decades while the overall ice extent in the Antarctic has increased slightly. However, this small Antarctic increase is actually the result of much larger regional increases and decreases, which are now shown to be caused by wind-driven changes. In places, increased northward winds have caused the sea-ice cover to expand outwards from Antarctica. The Arctic Ocean is surrounded by land, so changed winds cannot cause Arctic ice to expand in the same way.

Dr Ron Kwok, JPL says, “The Antarctic sea ice cover interacts with the global climate system very differently than that of the Arctic, and these results highlight the sensitivity of the Antarctic ice coverage to changes in the strength of the winds around the continent.”

There has been contrasting climate change observed across the Antarctic in recent decades. The Antarctic Peninsula has warmed as much as anywhere in the Southern Hemisphere, while East Antarctica has shown little change or even a small cooling around the coast. The new research improves understanding of present and future climate change. It is important to distinguish between the Antarctic Ice Sheet - glacial ice - which is losing volume, and Antarctic sea ice - frozen seawater - which is expanding.

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This research was funded by the Natural Environment Research Council (NERC) and National Aeronautics and Space Administration (NASA).

The paper 'Wind-driven trends in Antarctic sea ice motion' by Paul R. Holland of British Antarctic Survey and Ron Kwok of the Jet Propulsion Laboratory, California, USA is published in Nature Geoscience this week.

Stunning broadcast-quality footage and stills of Antarctica, and the embargoed Nature paper are available from the BAS & NASA Press Offices or to download at: ftp://ftp.nerc-bas.ac.uk/pub/photo/Images_PR12/ (credit to British Antarctic Survey)

An animation showing the extent of sea ice surrounding Antarctica throughout the year is also available to download at: ftp://ftp-projects.zmaw.de/seaice/AMSR-E_ASI_IceConc/SeaIceAnimations/AntarcticIceConcAMSRE-ASI-2002_06-2007_09.mpg (credit to Dr Gunnar Spreen at the Norwegian Polar Institute)

Sea ice. Floating sea ice caps the ocean around the Antarctic and although only 1-2 m thick, it provides effective insulation between the frigid Antarctic atmosphere and the relatively warm ocean below. By the end of winter the ice covers an area of 19 million square kilometres, more than doubling the size of the continent.

Glacier - a 'river of ice' fed by the accumulation of snow. Glaciers drain ice from mountains to lower levels, where the ice either melts, breaks away into the sea as icebergs, or feeds into an ice shelf.

Ice sheet - the huge mass of ice, up to 4 km thick that covers bedrock in Antarctica or Greenland. It flows from the centre of the continent towards the coast where it feeds ice shelves.

Ice shelf - the floating extension of the grounded ice sheet. Composed of freshwater ice that originally fell as snow, either in situ or in land and brought to the ice shelf by glaciers. As they are already floating, any disintegration will have no direct impact on sea level. Sea level will rise only if the ice held back by the ice shelf flows more quickly onto the sea.

British Antarctic Survey (BAS), a component of the Natural Environment Research Council (NERC), delivers and enables world-leading interdisciplinary research in the Polar Regions. Its skilled science and support staff based in Cambridge, Antarctica and the Arctic, work together to deliver research that uses the Polar Regions to advance our understanding of Earth as a sustainable planet. Through its extensive logistic capability and know-how BAS facilitates access for the British and international science community to the UK polar research operation. Numerous national and international collaborations, combined with an excellent infrastructure help sustain a world leading position for the UK in Antarctic affairs. For more information visit: www.antarctica.ac.uk

The Natural Environment Research Council (NERC) is the UK's main agency for funding and managing world-class research, training and knowledge exchange in the environmental sciences. It coordinates some of the world's most exciting research projects, tackling major issues such as climate change, environmental influences on human health, the genetic make-up of life on earth, and much more. NERC receives around £320 million a year from the government's science budget, which it uses to fund independent research and training in universities and its own research centres: www.nerc.ac.uk

NASA's Jet Propulsion Laboratory is a U.S. federally funded research and development facility managed by the California Institute of Technology for the National Aeronautics and Space Administration (NASA). JPL manages robotic spacecraft in the exploration of Earth, the solar system and the universe.